Liquid crystal displays (LCDs) require an external illumination source such as a backlighting device since the LCDs do not themselves emit light. Traditional backlighting devices include a narrow fluorescent tube that serves as a light source to input “white” color light into one of the sides of a light guide panel (also known as “light pipe panel”), which is positioned behind a LCD. The light from the fluorescent tube is internally reflected in the light guide panel and selectively emitted from the top surface of the wave guide panel toward the LCD, providing illuminating light for the LCD
With technological advancements in light emitting diodes (LEDs), the fluorescent tubes in traditional backlight devices are being replaced with LEDs. Some of the advantages of LEDs over fluorescent tubes include longer operating life, lower power consumption, and smaller in size. However, LEDs generally have narrow emission spectrum (approximately +/−10 nm). As an example, a blue InGaN LED may generate light with wavelength of 470 nm +/−10 nm. As another example, a green InGaN LED may generate light with wavelength of 510 nm +/−10 nm. As another example, a red AlInGaP LED may generate light with wavelength of 630 nm +/−10 nm. Due to the narrow-band emission characteristics, different emission types of monochromatic LEDs (e.g., red, green and blue LEDs) must be used together in a backlighting device to provide the white color illuminating light for a LCD. Alternatively, the original light emitted from monochromatic LEDs must be partially or almost completely converted to different wavelength light through photoluminescence, e.g., fluorescence, to provide the white color illuminating light.
Between these two approaches for producing white color illuminating light using monochromatic LEDs, the latter approach is generally preferred over the former approach. In contrast to the latter approach of using photoluminescence, the former approach of using different emission types of LEDs requires a more complex driving circuitry since the different emission types of LEDs have different operating voltage requirements. In addition, different emission types of LEDs tend to degrade differently over their operating lifetime, which makes color control over an extended period difficult using this approach.
A concern with the latter approach of using photoluminescence is that many of the phosphors that are currently available to produce white color illuminating light result in output light having lower-than-optimal Color Rendering Index (CRI).
In view of this concern, there is a need for a device and method for providing white color illuminating light using photoluminescence that can potentially increase the CRI of the illuminating light.